One of my friends couldn't join at my and other player session, so the problem was with my friend. We try all the solucions that we found here but what he did to fix that problem was reinstall the plutonium.exe launcher. For some reason his plutonium didn't let him join us. So go at plutonium download site, download button and maybe it can fix your problems.
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JPMachado04
@JPMachado04